The present invention relates to arrays of dielectric resonator antennas (DRAs) in which the patterns of the individual DRA elements may be electronically steered in synchronism with the array pattern.
Since the first systematic study of dielectric resonator antennas (DRAs) in 1983 [LONG, S. A., McALLISTER, M. W., and SHEN, L. C.: xe2x80x9cThe Resonant Cylindrical Dielectric Cavity Antennaxe2x80x9d, IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R. K. and BHARTIA, P.: xe2x80x9cDielectric Resonator Antennasxe2x80x94A Review and General Design Relations for Resonant Frequency and Bandwidthxe2x80x9d, International Journal of Microwave and Millimetre-Wave Computer-Aided Engineering, 1994, 4, (3), pp 230-247].
The majority of configurations reported to date have used a slab of dielectric material mounted on a ground plane excited by either an single aperture feed in the ground plane [ITTIPIBOON, A. MONGIA, R. K., ANTAR, Y. M. M., BHARTIA, P. and CUHACI, M: xe2x80x9cAperture Fed Rectangular and Triangular Dielectric Resonators for use as Magnetic Dipole Antennasxe2x80x9d, Electronics Letters, 1993, 29, (23), pp 2001-2002] or by a single probe inserted into the dielectric material [McALLISTER, M. W., LONG, S. A. and CONWAY G. L.: xe2x80x9cRectangular Dielectric Resonator Antennaxe2x80x9d, Electronics Letters, 1983, 19, (6), pp 218-219]. Direct excitation by a transmission line has also been reported by some authors [KRANENBURG, R. A. and LONG, S. A.: xe2x80x9cMicrostrip Transmission Line Excitation of Dielectric Resonator Antennasxe2x80x9d, Electronics Letters, 1994, 24, (18), pp 1156-1157].
The concept of using a series of these single feed DRAs to build an antenna array has already been explored. For example, an array of two cylindrical single-feed DRAs has been demonstrated [CHOW, K. Y., LEUNG. K. W., LUK, K. M. AND YUNG, E. K. N.: xe2x80x9cCylindrical dielectric resonator antenna arrayxe2x80x9d, Electronics Letters, 1995, 31, (18), pp 1536-1537] and then extended to a square matrix of four DRAs [LEUNG, K. W., LO, H. Y., LUK, K. M. AND YUNG, E. K. N.: xe2x80x9cTwo-dimensional cylindrical dielectric resonator antenna arrayxe2x80x9d, Electronics Letters, 1998, 34, (13), pp 1283-1285]. A square matrix of four cross DRAs has also been investigated [PETOSA, A., ITTIPIBOON, A. AND CUHACI. M.: xe2x80x9cArray of circular-polarized cross dielectric resonator antennasxe2x80x9d, Electronics Letters, 1996, 32, (19), pp 742-1743]. Long linear arrays of single-feed DRAs have also been investigated with feeding by either a dielectric waveguide [BIRAND, M. T. AND GELSTHORPE, R. V.: xe2x80x9cExperimental millimetric array using dielectric radiators fed by means of dielectric waveguidexe2x80x9d, Electronics Letters, 1983, 17, (18), pp 633-635] or a microstrip [PETOSA. A., MONGIA, R. K., ITTIPIBOON, A. AND WIGHT, J. S.: xe2x80x9cDesign of microstrip-fed series array of dielectric resonator antennasxe2x80x9d, Electronics Letters, 1995, 31, (16), pp 1306-1307]. This last research group have also found a method of improving the bandwidth of microstrip-fed DRA arrays [PETOSA, A., ITTIPIBOON, A., CUHACI, M. AND LAROSE, R.: xe2x80x9cBandwidth improvement for microstrip-fed series array of dielectric resonator antennasxe2x80x9d, Electronics Letters, 1996, 32, (7), pp 608-609]. It is important to note that none of these publications have discussed the concept of multi-feed DRAs or the concept of array element steering.
Earlier work by the present inventors [KINGSLEY, S. P. and O""KEEFE, S. G., xe2x80x9cBeam Steering and Monopulse Processing of Probe-Fed Dielectric Resonator Antennasxe2x80x9d, IEE Proceedingsxe2x80x94Radar, Sonar and Navigation, 146, 3, 121-125, 1999] shows how several spatially separated feeds can be used to drive a single circular slab of dielectric material so as to produce an antenna with several beams facing in different directions. The simultaneous excitation of several feeds means that the DRA can have electronic beamsteering and direction finding capabilities. This work is also disclosed in the present applicants U.S. patent application Ser. No. 09/431,548 entitled xe2x80x9cSteerable-beam multiple-feed dielectric resonator antennaxe2x80x9d, the disclosure of which is incorporated into the present application by reference.
The present application extends the previous work of Kingsley and O""Keefe by considering the properties and benefits of arrays composed of many such multi-feed DRAs. A wide range of array geometries is considered.
An antenna array is a collection of (often evenly spaced) simple elements such as monopoles, dipoles, patches, etc. The arrangement of elements to form the array may be linear, 2-D, in a circle, etc. and the shape of 2-D arrays may be rectangular, circular, oval, etc. In an array, each individual element has a broad radiation pattern but when they are combined together, the array as a whole has a much narrower radiation pattern. More importantly, by feeding the elements with different phases or time delays, the array pattern can be steered electronically. This is a most useful facility in radar and communications.
It is important to distinguish between the various radiation patterns referred to in the present application. Firstly, each element of the array has its own notional radiation pattern when considered in isolation. This element pattern may be considered to be analogous to the diffraction pattern of one of the light sources in a Young""s slits interference demonstration. Secondly, the array as a whole has a notional radiation pattern, known as the array factor, which is the sum of the idealised isotropic element patterns, and which may be considered to be analogous to the interference pattern in a Young""s slits demonstration. Finally, the actual radiation pattern formed by the antenna array, known as the antenna pattern, is the product of the element patterns and the array factor. Each of the element pattern, array factor and antenna pattern may be considered to have a direction in which transmission/reception has a maximum gain, and embodiments of the present invention seek to steer these directions in useful ways.
The radiation patterns of the individual elements of an array are fixed so that when the array factor faces straight ahead (on boresight), the resultant antenna pattern has the benefit of the full gain of each individual element. In fact, the gain of the array is the sum of the gain of the elements. However, when the array factor is steered off boresight, the gain can fall because the array factor is moving outside the pattern of the individual elements. The only time this is not true is when the elements are omnidirectional in the plane of the array (such as monopoles), but as these are usually low gain elements there still remains a problem of low gain overall.
Embodiments of the present invention seek to provide an array of dielectric resonator antenna elements, where each element has several energy feeds connected in such a way that the radiation pattern of each element can be steered. One method of electronically steering an antenna element pattern is to have a number of existing beams and to switch between them or, alternatively, to combine them so as to achieve the desired beam direction. The general concept of deploying a plurality of probes within a single dielectric resonator antenna, as pertaining to a cylindrical geometry, is described in the paper KINGSLEY, S. P. and O""KEEFE, S. G., xe2x80x9cBeam Steering and Monopulse Processing of Probe-Fed Dielectric Resonator Antennasxe2x80x9d, IEE Proceedingsxe2x80x94Radar, Sonar and Navigation, 146, 3, 121-125, 1999, the disclosure of which is incorporated into the present application by reference.
It has been noted by the present applicants that the results described in the above reference apply equally to DRAs operating at any of a wide range of frequencies, for example from 1 MHz to 100,000 MHz and even higher for optical DRAs. The higher the frequency in question, the smaller the size of the DRA, but the general beam patterns achieved by the probe/aperture geometries described hereinafter remain generally the same throughout any given frequency range. Operation at frequencies substantially below 1 MHz is also possible, using dielectric materials with a high dielectric constant.
According to the present invention, there is provided an array of dielectric resonator antenna elements, each element being composed of at least one dielectric resonator and a plurality of feeds for transferring energy into and from the elements, wherein the feeds of each element are activatable either individually or in combination so as to produce at least one incrementally or continuously steerable element beam which may be steered through a predetermined angle, characterised in that, during operation of the array, the feeds of the elements are activated such that the element beams from the different elements are steered in synchrony with each other, and in that the element beams, when combined, interact so as to form at least one array beam which is steered in synchrony with the element beams.
The array may be provided with electronic circuitry adapted to activate the feeds either individually or in combination so as to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle.
The array may additionally be provided with further electronic circuitry adapted to activate each of the antenna elements with a pre-determined phase shift or time delay so as to generate an array factor which may be steered through a predetermined angle. For example, for a given array factor direction (which here is the same as the antenna beam direction), each element may be fed with a different phase or time delay (and, in practice, a different amplitude) so that when the element patterns are added together, they give rise to an antenna pattern in a predetermined direction. For a different antenna beam direction, the phases and amplitudes of the element feeds will be different.
By providing an array of steerable DRAs, the present invention seeks to enable the individual element patterns to be steered in synchronism with the array factor as a whole, thereby forming an array having maximum or at least improved element gain for a given array factor direction.
The elements of the array may be arranged in a substantially linear formation, and may be arranged side by side so as to provide azimuth beamsteering or one on top of the other so as to provide elevation as well as azimuth beamsteering. The elements may or may not be evenly spaced, depending on requirements, and the linear array may be arranged so as to be conformal to a curved or distorted surface. This latter feature has potentially important implications in, for example, communications on aircraft. For example, by conforming a linear array of elements to the fuselage of an aircraft and by arranging for the element beam patterns all to face the same way regardless of the actual orientation of the elements on the fuselage, it is possible to match an array beam pattern with the element beam pattern so as to improve gain. Furthermore, a dielectric lens may be provided so as to improve control of azimuth and/or elevation beamsteering.
Alternatively, the elements of the array may be disposed in a ring-like formation, such as a circle, or may be disposed more generally in at least two dimensions across a surface. The elements may or may not be evenly spaced, and may, for example, be in the form of a regular lattice. As discussed above, the surface in which the elements are disposed may be conformed to a curved or distorted surface, such as the fuselage of an aircraft, and the elements may be individually controlled so that the element beam patterns all face the same way regardless of the individual physical orientations of the elements themselves. Furthermore, a dielectric lens may be provided so as to improve control of azimuth and/or elevation beamsteering
Alternatively, the elements of the array may be arranged as a three dimensional volumetric array, the array as a whole having an outer envelope in the form of a regular solid (e.g. sphere, tetrahedron, cube, octahedron, icosahedron or dodecahedron) or an irregular solid. The elements may or may not be evenly spaced, and may, for example, be in the form of a regular lattice. The volumetric array may be formed as a combination of linear and/or surface arrays stacked one on top of the other so as to allow both azimuth and elevation beamsteering. Furthermore, a dielectric lens may be provided so as to improve control of azimuth and/or elevation beamsteering.
Beamsteering in elevation is achieved by stacking the DRA elements on top of each other, or by forming a stack of DRA arrays, and by energising the elements appropriately. For example, in a vertical stack of cylindrical multi-probe elements, each element on its own can steer an element beam in azimuth, and it is possible to feed the probes so that all of the elements form element beams which face in the same direction. When combined, these element beams form a horizontal beam in the chosen direction which is smaller in elevation than the elevation pattern of a single element. By changing the phasing, for example, between the element feeds, it is possible to move the combined beam up and down in elevation. In a more complex system, there may be provided a vertical stack of linear element arrays.
Advantageously, the antenna array as a whole is adapted to produce at least one incrementally or continuously steerable beam, which may be steered through a complete 360 degree circle.
Advantageously, each individual element of the antenna array is also adapted to produce at least one incrementally or continuously steerable beam, which may be steered through a complete 360 degree circle.
Advantageously, there is additionally or alternatively provided electronic circuitry to combine the feeds of each individual element of the antenna array such that the element pattern is steered in angle in synchronism with the antenna array pattern.
Advantageously, there is additionally or alternatively provided electronic circuitry to provide at least two feeds to each individual element of the antenna array such that, when the array is used to form at least two array factors simultaneously, the elements are activatable so as to form at least two element beams simultaneously which are steerable in synchronism with the antenna pattern (which is the sum of the at least two array factors).
Generally, the at least two array factors together form an antenna pattern having two main lobes.
When a conventional antenna array is used to form at least two beams simultaneously, then at least two sets of phases and amplitudes for the elements must be combined by driving each element through one (or more) power splitter/combiners which are large, lossy devices. Embodiments of the present invention can achieve the same result by simply connecting one set of phases and amplitudes to one particular feed to each DRA element and another set of phases and amplitudes up a different feed to each element.
The feed to each element may include a cable, fibre optic connection, printed circuit track or any other transmission line technique, and these may be of predetermined different effective lengths so as to insert different time delays in the feed to each element, thus providing beamsteering control. The delays may be controlled and varied by controlling and varying the effective lengths of the transmission lines, either electrically, electronically or mechanically, for example by switching additional lengths of transmission line in and out of the base transmission lines.
Alternatively or in addition, beamsteering may be effected by individually adjusting the phase of the feed to each element, for example by including diode phase shifters, ferrite phase shifters or other types of phase shifters into the transmission lines. Additional control may be achieved by varying the amplitude of signals in the transmission lines, for example by including attenuators therein.
The feed mechanisms to the elements may incorporate a resistive beamforming matrix of phase shifters so as to insert different phase delays in the feed to each element. Alternatively or in addition, the feed mechanisms to the elements may incorporate a matrix of hybrids, such as a Butler matrix, so as to form a plurality of beams from a plurality of elements. A Butler matrix is a parallel RF beam-forming network that forms N contiguous beams from an N-element array. The network makes use of directional couplers, fixed phase differences and transmission lines. It is lossless apart from the insertion loss of these components. Other types of RF beamforming networks also exist.
Alternatively or in addition, xe2x80x9cweightingxe2x80x9d or xe2x80x9cwindowxe2x80x9d function may be applied electronically or otherwise to the feeds to the elements so as to control array factor sidelobes. Exciting all elements equally gives a uniform aperture distribution that results in high array factor sidelobe levels. Applying a window function, such that the elements towards the edge of the array contribute less to the array factor than those at the centre, can reduce these sidelobe levels.
Alternatively or in addition, an xe2x80x9cerrorxe2x80x9d or xe2x80x9ccorrectionxe2x80x9d function may be applied electronically or otherwise to the feeds of the elements so as to control embedded element, mutual coupling, surface wave and other perturbing effects. Simple array theory assumes that all the elements behave identically. However, those disposed toward the edge of an array may behave differently to those nearer the centre, because of the reasons given above. For example, an element at the centre experiences mutual coupling to the elements either side, but an element at the edge has no neighbour on one side. These error effects can be measured and corrected for by applying a correction factor.
Each element of the array may be connected to a single beamforming mechanism so as to produce a single array factor, or to a plurality of beamforming mechanisms so as simultaneously to produce a plurality of array factors.
The elements of the array may be disposed so as to permit various polarisations to be achieved, such as vertical, horizontal, circular or any other polarisation, including switchable or otherwise controllable polarisations. For example, MONGIA, R. K., ITTIPIBOON, A., CUHACI, M. and ROSCOE D.: xe2x80x9cCircular Polarised Dielectric Resonator Antennaxe2x80x9d, Electronics Letters, 1994, 30, (17), pp 1361-1362; and DROSSOS, G., WU, Z. and DAVIS, L. E.: xe2x80x9cCircular Polarised Cylindrical Dielectric Resonator Antennaxe2x80x9d, Electronics Letters, 1996, 32, (4), pp 281-283.3, 4, the disclosures of which are incorporated into the present application by reference, describe how two probes fed simultaneously in a circular cross-section dielectric slab and installed on radials at 90xc2x0 to each other can create circular polarisation when fed in anti-phase. Furthermore, DROSSOS, G., WU, Z. and DAVIS. L. E.: xe2x80x9cSwitchable Cylindrical Dielectric Resonator Antennaxe2x80x9d, Electronics Letters, 1996, 32, (10), pp 862-864, the disclosure of which is also incorporated into the present application by reference, describes how polarisation may be achieved by switching the probes on and off.
Advantageously, there is additionally or alternatively provided electronic circuitry or computer software such that when digital beamforming techniques are used, the feeds of each individual element of the antenna array are controlled in such a way that the element pattern is steered in angle in synchronism with the array factor.
When each element of the array is connected to a separate transmitter module, a separate receiver module or a separate transmitter/receiver module, then digital beamforming techniques may be used to form steerable array factors of any desired shape which are steerable both in azimuth as well as in elevation.
With a conventional array (analogue beamsteering), a single transmitter or receiver is distributed to each element with the appropriate phase and amplitude modifications along each path. With digital beamforming, each element has its own transmitter or receiver and is instructed by a computer to form the appropriate phase and amplitude settings. In the receiving case, each receiver has its own A/D converter, the outputs of which can be used to form almost any desired beam shape, many different beams simultaneously, or even be stored in the computer and the beams formed some time later.
Many such array factors may be formed simultaneously by digital beamforming techniques through appropriate electronic or software control. Such array factors may contain one or more nulls in order to cancel interference, multipath or other unwanted signals in given directions. Alternatively, the DRA element pattern may be arranged so as to cancel some or all of the unwanted signals. For example, where a digital beamforming array has N elements then it generally has Nxe2x88x921 degrees of freedom, and so may be able to null out jamming signals from Nxe2x88x921 different directions. In embodiments of the present invention, each DRA element may also have at least one null in its radiation pattern, and this may be used to null out jamming signals from at least one additional direction. Digitally beamformed array patterns may be formed on-line in real time or, in the case of recorded received data, off-line at a later time.
Preferably, the array pattern steering and the synchronous element pattern steering is carried out through a complete 360 degree circle.
In one embodiment of the present invention, the dielectric resonator elements may be divided into segments by conducting walls provided therein, as described, for example, in U.S. Ser. No. 09/431,548 and in more detail in the present applicant""s co-pending UK patent application no 0005766.1 filed on 11th Mar. 2000 and International patent application no PCT/GB01/00929, filed on 2nd Mar. 2001, both entitled xe2x80x9cMulti-segmented dielectric resonator antennaxe2x80x9d, the full disclosures of which are incorporated into the present application by reference.
In a further embodiment of the present invention, there may additionally be provided at least one internal or external monopole antenna or any other antenna possessing a circularly symmetrical pattern about a longitudinal axis, which is combined with at least one of the dielectric resonator antenna elements so as to cancel out backlobe fields or to resolve any front-to-back ambiguity which may occur with a dielectric resonator antenna having a cosine or figure-of-eight radiation pattern. The monopole or other circularly symmetrical antenna may be centrally disposed within the dielectric resonator element or may be mounted thereupon or therebelow and is activatable by the electronic circuitry. In embodiments including an annular resonator with a hollow centre, the monopole or other circularly symmetrical antenna may be located within the hollow centre. A xe2x80x9cvirtualxe2x80x9d monopole may also be formed by an electrical or algorithmic combination of any of the actual feeds, preferably a symmetrical set of feeds.
The dielectric elements or the dielectric resonators making up the elements may be formed of any suitable dielectric material, or a combination of different dielectric materials, having an overall positive dielectric constant k. Different elements or resonators may be made out of different materials having different dielectric constants k, or they may all be made out of the same material. Equally, the elements or resonators may all have the same physical shape or form, or may have different shapes or forms as appropriate. In preferred embodiments, k is at least 10 and may be at least 50 or even at least 100. k may even be very large e.g. greater than 1000, although available dielectric materials tend to limit such use to low frequencies. The dielectric material may include materials in liquid, solid, gaseous or plasma states, or any intermediate state. The dielectric material may be of lower dielectric constant than a surrounding material in which it is embedded.
The feeds may take the form of conductive probes which are contained within or placed against the dielectric resonators, or a combination thereof, or may comprise aperture feeds provided in a grounded substrate. Aperture feeds are discontinuities (generally rectangular in shape) in a grounded substrate underneath the dielectric material and are generally excited by passing a microstrip transmission line beneath them. The microstrip transmission line is usually printed on the underside of the substrate. Where the feeds take the form of probes, these may be generally elongate in form. Examples of useful probes include thin cylindrical wires which are generally parallel to a longitudinal axis of the dielectric resonator. Other probe shapes that might be used (and have been tested) include fat cylinders, non-circular cross sections, thin generally vertical plates and even thin generally vertical wires with conducting xe2x80x9chatsxe2x80x9d on top (like toadstools). Probes may also comprise metallised strips placed within or against the dielectric, or a combination thereof. In general, any conducting element within or against the dielectric resonator, or a combination thereof, will excite resonance if positioned, sized and fed correctly. The different probe shapes give rise to different bandwidths of resonance and may be disposed in various positions and orientations (at different distances along a radius from the centre and at different angles from the centre, as viewed from above) within or against the dielectric resonator or a combination thereof, so as to suit particular circumstances. Furthermore, there may be provided probes within or against the dielectric resonator, or a combination thereof, which are not connected to the electronic circuitry but instead take a passive role in influencing the transmit/receive characteristics of the dynamic resonator antenna, for example, by way of induction.
Generally, where the feed comprises a monopole feed, then the appropriate dielectric resonator element or dielectric resonator must be associated with a grounded substrate, for example by being disposed thereupon or separated therefrom by a small air gap or a layer of another dielectric material. Alternatively, where the feed comprises a dipole feed, then no grounded substrate is required. Embodiments of the present invention may use monopole feeds to dielectric elements or resonators associated with a grounded substrate, and/or dipole feeds to dielectric elements or resonators not having an associated grounded substrate. Both types of feed may be used in the same antenna.
Where a grounded substrate is provided, the dielectric resonators may be disposed directly on, next to or under the grounded substrate, or a small gap may be provided between the resonators and the grounded substrate. The gap may comprise an air gap, or may be filled with another dielectric material of solid, liquid or gaseous phase.
The antenna array of the present invention may be operated with a plurality of transmitters or receivers, the terms here being used to denote respectively a device acting as a source of electronic signals for transmission by way of the antenna array or a device acting to receive and process electronic signals communicated to the antenna array by way of electromagnetic radiation. The number of transmitters and/or receivers may or may not be equal to the number of elements being excited. For example, a separate transmitter and/or receiver may be connected to each element (i.e. one per element), or a single transmitter and/or receiver to a single element (i.e. a single transmitter and/or receiver is switched between elements). In a further example, a single transmitter and/or receiver may be (simultaneously) connected to a plurality of elements. By continuously varying the feed power between the elements, the beam and/or directional sensitivity of the antenna array may be continuously steered. A single transmitter and/or receiver may alternatively be connected to several non-adjacent elements. In yet another example, a single transmitter and/or receiver may be connected to several adjacent or non-adjacent elements in order to produce an increase in the generated or detected radiation pattern, or to allow the antenna array to radiate or receive in several directions simultaneously.
The array of elements may simply be surrounded by air or the like, or may be immersed in a dielectric medium having a permittivity between that of air and that of the elements themselves. In the latter case, the effective separation distance between the elements is reduced, and the dielectric medium can therefore be arranged to act as a dielectric lens. For example, if an array of any type is immersed in a dielectric medium having a relative permittivity Er, then the size of the array can be reduced by Er.
By seeking to provide an antenna array composed of a plurality of dielectric resonator elements, each capable of generating multiple beams which can be selected separately or formed simultaneously and combined in different ways at will, embodiments of the present invention may provide the following advantages:
i) By choosing to drive different probes or apertures, the antenna array and each array element can be made to transmit or receive in one of a number of preselected directions (in azimuth, for example). This has the advantage that the gain of the array is always maximised by having maximum element gain. With a conventional antenna array (composed of dipoles, for example), as the array factor is steered away from the straight ahead xe2x80x98boresightxe2x80x99 position, the gain begins to fall because the array factor is steered outside the element pattern. A conventional array of dipoles, for example, cannot be steered through 360 degrees in the plane of the dipoles because at some point, usually at a steering angle of 90 degrees, the array factor falls into a null of the element pattern.
ii) By sequentially switching round the element feeds, and simultaneously switching round the array beam pattern, the resultant antenna radiation pattern can be made to rotate incrementally in angle. Such beam-steering has obvious applications for radio communications, radar and navigation systems.
iii) By combining two or more feeds simultaneously, element beams can be formed in any arbitrary azimuth direction to match an array factor formed in any arbitrary direction, thus giving more precise control over the beamforming process whilst maintaining improved or maximum antenna gain.
iv) By electronically continuously varying the power division/combination of two or more feeds simultaneously, element beams can be steered continuously in synchronism with an array factor that is being steered continuously.
v) When at least two beams in different directions are formed simultaneously with the array, then the plurality of feeds in the antenna elements can be so disposed as to form more than one beam at once to match the array factor.
vi) The addition of an internal or external monopole antenna or other antenna possessing a circularly symmetrical radiation pattern about a longitudinal axis can be used to cancel or reduce a backlobe of the antenna array, thereby resolving any front-to-back ambiguity in, for example, a linear array.